1. Technical Field of the Invention
The present invention generally relates to the wireless telecommunications field and, in particular, to a radio receiver and method for preloading a channel filter with an average DC offset before filtering a signal within the channel filter.
2. Description of Related Art
A mobile phone incorporates many components which can include a radio receiver configured as a homodyne receiver or heterodyne receiver. Unfortunately, the above-mentioned radio receivers generate an undesirable DC offset that adversely affects or increases a transient settling time in time division multiple access (TDMA) based communication systems. Transient settling time or start-up time is the length of time required within a filter to reduce the undesirable DC offset to an acceptable level before being able to output a desired signal from the radio receiver. The traditional solution used to reduce the transient settling time within the filter is to compensate for the undesirable DC offset by adding complex DC offset compensation circuitry to an analog section of the radio receiver.
An example of the traditional radio receiver incorporating the complex analog DC offset compensation circuitry is briefly discussed below and disclosed in Baker et al. U.S. Pat. No. 5,724,653.
Referring to FIG. 1 (PRIOR ART), there is illustrated a block diagram of the traditional radio receiver 100 incorporating complex analog DC offset compensation circuitry 102 described in the ""653 patent. The general architecture of the radio receiver 100 (e.g., heterodyne receiver) is well known in the industry and as such is not described in great detail herein.
Basically, the traditional radio receiver 100 includes an antenna 104 for receiving an input radio signal from a transmitter 106. The received input radio signal is amplified through a low noise amplifier 108 and coupled to a first mixer 110, via a first switch 112. The first mixer 110 connects to a first local oscillator 114 by way of a second switch 116 and outputs an intermediate frequency (IF) signal 117 related to the received input radio signal. The first switch 112 and the second switch 116 include some of the components associated with the DC offset compensation circuitry 102.
The IF signal 117 is filtered by a bandpass filter 118 coupled to the first mixer 110, and amplified by a variable gain amplifier 120 coupled in series with the bandpass filter. A capacitor 122 couples the variable gain amplifier 120 to a demodulation circuit configured to demodulate the IF signal 117 into a baseband inphase (I) signal 124 and a baseband quadrature (Q) signal 126. More specifically, the demodulation circuit includes a second local oscillator 128 connected to a second mixer 130, third mixer 132 and phase shifter 133 that collectively operate to convert the IF signal 117 into the baseband I signal 124 and the baseband Q signal 126, reactively.
The demodulation circuit further includes a first DC correction circuit 134 and a second DC correction circuit 136 representing the remaining components of -the DC offset compensation circuitry 102. The first and second DC correction circuits 134 and 136 each include a low pass filter 138 for reducing noise bandwidth and filtering erroneous samples associated with the baseband I signal 124 and the baseband Q signal 126.
The DC offset compensation circuitry 102 is configured such that each of the first and second DC correction circuits 134 and 136 are adapted to store a DC offset prior to the received input radio signal being coupled into the first mixer 110. In other words, the first and second switches 112 and 116 are opened to prevent the received input radio signal from entering the first mixer 110 when the DC offsets are being stored in the first and second DC correction circuits 134 and 136.
Thereafter, each of the first and second DC correction circuits 134 and 136 operate to subtract the stored DC offset from the corresponding baseband I signal 124 and baseband Q signal 126 received upon closing the first and second switches 112 and 116. The baseband I signal 124 and baseband Q signal 126 are then amplified by respective amplifiers 140 and converted to digital baseband signals by respective analog-to-digital (A/D) convertors 142. The digital baseband signals are filtered by digital filters 144 and then digitally demodulated by a digital demodulator 146 to output a desired signal 148. of course, the timing required to open and close the first and second switches 112 and 116 combined with the operations of storing and subtracting of the DC offsets by the first and second DC correction circuits 134 and 136 of the traditional radio receiver 100 is a complex way to compensate for DC offsets. Accordingly, there is a need for a radio receiver and method that effectively compensates for undesirable DC offsets in a less complex manner.
The present invention is A method and radio receiver that effectively compensates for an undesirable DC offset in a digital section of the radio receiver by preloading a filter with an average DC offset before routing a signal through the filter. More specifically, the radio receiver includes an antenna for receiving a radio signal, and an analog section for demodulating the received radio signal into at least one baseband signal. The radio receiver also includes an analog-to-digital section for converting the at least one baseband signal into at least one digital baseband signal. The radio receiver further, includes a preloading system for calculating an average DC offset using a predetermined number of symbols from the at least one digital baseband signals and for preloading a filter with the calculated average DC offset prior to filtering the at least one digital baseband signal in the filter.
In accordance with the present invention, there is provided a method and radio receiver that reduces the current consumption of the radio receiver.
Also in accordance with the present invention, there is provided a method and radio receiver that reduces a transient settling time in a filter.
Further in accordance with the present invention, there is provided a method and radio receiver that effectively suppresses interference attributable to a filter in an adjacent timeslot.